The probes are understood to be looking at election expenses of ten sitting MPs, so far.

The investigations centre on whether costs to transport and accommodate bus-loads of Tory activists to marginal seats should have been declared by candidates.

It follows a four month investigation by Channel 4 News into campaign spending by the Conservative Party in last year’s General Election and three key by-elections.

One force, Greater Manchester Police, confirmed it had already been granted an extensions by the courts to extend the one-year time limit for prosecutions under election law.

A further three forces said they would be doing the same in coming weeks.

Last night legal experts said it is the first time in British history that extensions had been sought to time limits for prosecutions under election law, and showed the scale of the operation.

One of Britain’s leading election lawyers Gavin Millar QC, of Matrix Chambers said: “We haven’t seen a case like this before – where a group of police forces have used this power.

“I don’t know of a case where it’s been used even in a single case.

“But the idea of a group of police forces coming together to coordinate an investigation – and each one getting the extension of time to look into the election in their area is unheard of.

“And it means that they are taking seriously as police service the issues that were raised by the original Channel 4 News report, and the issues that people like me have been worried about for a long time about malpractice in marginal seats involving the major political parties in terms of how they spend and what they’re prepared to admit having spent.”

The forces that have confirmed they are investigating are Devon and Cornwall, Cheshire, West Yorkshire, Greater Manchester Police, Gloucestershire, Northamptonshire, and Staffordshire.

The MPs we understand are being looked at are Amanda Milling, for Cannock Chase; Michael Ellis for Northampton North; Stuart Andrew for Pudsey, Horsforth & Aireborough; David Nuttall for Bury North; North Cornwall for Scott Mann; George Eustace for Cambourne and Redruth; Kevin Foster for Torbay; Oliver Colville for Plymouth Sutton and Devenport; and Graham Evans for Weaver Vale.

Gloucestershire Police has not said whether their investigation is looking at Cheltenham MP Alex Chalk or Stroud MP Neil Carmichael.

So far, the Channel 4 News investigation has found almost £200,000 of seemingly undeclared election expenses in 33 constituency elections carried out in 2014 and 2015.

SOUTH THANET

In January, Channel 4 News revealed £19,151 WAS spent by the Conservatives in the South Thanet constituency during the 2015 General Election when the party fought to defeat UKIP leader Nigel Farage.

More than £17,000 of these hotel costs WERE declared as ‘national’ campaign expenses even though the Conservative workers who used the accommodation appeared to be involved in local campaigning for candidate Craig Mackinlay.

Election law says any money spent campaigning for a specific local candidate MUST be declared on the local spending return.

A smaller hotel bill for party workers in Thanet South – for £715 – appears not to have been declared at all.

However, if all these costs were local and had to be declared on the Conservative’s Thanet South spending return, the candidate would HAVE spent more than twice the £15,016 legal campaign spending limit.

THREE BY-ELECTIONS

In February, Channel 4 New revealed at least £99,112 worth of hotel receipts from three key by-elections in 2014 – Newark, Clacton and Rochester & Strood – that were seemingly never declared.

The hotel bookings included more than 1,200 nights of accommodation in hotels, with 770 bookings using the name or address of a single Conservative Party campaign worker.

If the costs had TO be declared on the local returns, all three Conservative candidates in each of the three key by-election contests against UKIP would have exceeded their £100,000 spending limits.

THE BATTLEBUS TOUR

In April, Channel 4 News revealed £64,127 was spent by the Tories to transport busloads of activists to campaign in marginal seats in the South West, North and Midlands.

The so-called Battlebus2015 tour sent four buses to 29 key marginals in the last ten days of the campaign, 26 of which were ultimately won by Conservative candidates.

The costs included £38,201 for hotel bills to accommodate the activists in regional ‘hub’ hotels during the tour, which were not declared by the Conservatives on either national or local spending returns.

In response to questions from Channel 4 News, the party stated that the omission was an “administrative error”.

The remaining costs, which covered the buses and additional staff costs, were declared as party spending on the Conservative’s national return.

However, evidence from documents and social media, appeared to show the activists on the Battlebuses ALSO campaigned for local candidates, and costs should have been declared on candidate returns.

Spending limits are legally set around £15,000 depending on the number of electors in the seats.

Once the costs for accommodation and transport are divided by each constituency visited, it could mean THAT additional costs would need to be included in the spending returns of each candidate – an extra £2,460 for the seats visited in the South West, an additional £2,045 for the seats in the Midlands, and an extra £2,154 for the seats in the North.

The additional costs could have pushed many over the legal spending limits.